The present invention relates to a titanium oxide compound for use in an electrode, which is useful as an active material for lithium secondary batteries, and a lithium secondary battery comprising the same.
Lithium secondary batteries have improved as power sources for cellular phones and notebook computers because of their high energy density. With recent advances in IT technology which have required a reduction in the size and weight of portable remote terminals, batteries used as power sources for these terminals have also been required to have a smaller size and a higher capacity. Moreover, based on their high energy density, lithium secondary batteries have begun to receive attention as power sources for electric vehicles or hybrid vehicles and as power sources for power storage.
As to negative electrode materials for conventional lithium batteries, carbon-based negative electrodes are commonly used, and lithium secondary batteries comprising the same are characterized by providing high voltage during discharge and having high energy density. However, such negative electrodes have low electric potential and hence will induce an increased risk of internal short circuit caused by lithium metal deposited upon rapid charge and also a risk of ignition caused by the internal short circuit. For this reason, studies have been conducted to develop lithium batteries with high safety and long life, which are designed to have high potential negative electrodes to reduce heat generation upon internal short circuit although the energy density is lowered, and are further designed to suppress the decomposition of an electrolyte. Among them, Li4Ti5O12 has an electric potential of 1.5 V on a lithium basis and is excellent in cycling characteristics because there is no volume change during charge and discharge. Thus, coin batteries comprising Li4Ti5O12 have been developed for practical use.
However, Li4Ti5O12 has a theoretical capacity of 175 mAh/g, which is as low as about a half of the electric capacity of carbon commonly used as a negative electrode material, and also has a drawback of reducing energy density when used in lithium secondary batteries. For this reason, there is a demand for a negative electrode material of high electric capacity having a voltage of 1.0 to 1.5 V vs. Li/Li+ in terms of safety and long life.
Under these circumstances, Patent Document 1 shows A2Ti3O7 (wherein A is one or two members selected from Na, Li and H) having the same crystal structure as that of Na2Ti3O7, while Patent Document 2 shows that titanate bronze-type titanium dioxide having an isotropic shape of micron size is useful as an electrode material. However, the discharge capacity shown in these documents is less than 180 mAh/g, which essentially does not greatly exceed the electric capacity of titanium-based negative electrodes conventionally known. Moreover, Patent Document 3 shows that tunnel-structured titanium oxide having a specific crystal structure (H2Ti12O25) achieves a discharge capacity of around 220 mAh/g. However, there is no information about particle size, and the effect of particle size is not clarified in this document. In other crystal structures, the effect of particle size is also not clarified.
The electric capacity of a lithium secondary battery is determined by the size of the particle surface through which lithium ions can enter or exit, the mobility of lithium ions across particles, and the number of sites at which lithium ions are held. The number of sites is determined by the crystal structure. For effective use of these sites, it is necessary to increase the specific surface area of particles through which lithium ions enter or exit the particles and to reduce the distance required for lithium ions to move into the particles. Thus, crystallite size is effective as a parameter representing the size of primary particles, i.e., migration length.    Patent Document 1: Japanese Patent Public Disclosure No. 2007-234233    Patent Document 2: Japanese Patent Public Disclosure No. 2008-117625    Patent Document 3: Japanese Patent Public Disclosure No. 2008-255000